This is a PCM-controlled secondary air injection system, which is used on turbocharged Forester models manufactured in 2006 and onwards. It functions by pumping in more air into the exhaust pipe during cold starts to help burn off unburned hydrocarbons. There are two secondary air combination valves and an electric air pump, all under the control of a PCM (Powertrain Control Module). The secondary air pump is located at the left front corner of the engine compartment. To replace it, disconnect the cable from the negative battery terminal and then take out electrical connector and hose from the pump; however three mounting bracket bolts should be removed also. In addition, this system has two secondary air combination valves. First, for replacing left valve you need to disconnect electrical connector, remove air duct, unbolt outlet pipe and remove valve. Next comes right valve that needs to be unplugged from intake manifold after having left one been removed. Then unbolt outlet pipe and remove it.